Reduction in reflection from glass



G. L. DlwnvuczK REDUCTION IN REFLECTION FROM GLASS FledlDeC. .'50, 1942 2 um; FSMu o wvo nm mMmzr a GM ww n :inventar Glenn L. Dammw Cttorneg REDUCTON EN REFLECTION FROM GLASS Glenn it. Dimmlck, Indianapolis, Ind., assignor to i Radio Corporation oi America, a corporation of Delaware Application December 30, 1942, Serial No. 470.1583

26 Claims.

pound of thorium and fluoride, probably thorium n exi-fluoride. t Due to the high index of this material, it did not appear at first that the material could be used. to produce a reiiection reducing iilm although it was extremely lhard and had high mechanical and chemical durability. The present application relates to a modication of the said multi-layer iilm to produce a minimum of reiiection from the glass surface and at the same time to retain the advantages of mechanical strength and chemical resistance. This is accomplished by applying lms in such relation of materials and thicknesses as to accomplish the renection reducing result without undesirabiy adecting the characteristics of the surface nlm or' the thorium compound,

My invention relates particularly to the reduction in reflection of light in the visible spectrum but it applies also to radiant energy outside the spectrum such as ultra violet and infra-red wavelengths. It will be understood in this specication that the term iight will include such other forms oi' radiant energy.

One object of the invention is to provide a surface coating for optical elements having an extremely lov.r redaction.

Another object of the invention is to provide asurface coating for optical elements having a very high mechanical strength.

Another object of the invention is to provide a. surface coating having a chemical resistance exceeding that of some types of optical glass.

Other and incidental objects of the invention will be apparent to those skilled in the art from i a reading of the following specification and an inspection of thev accompanying drawing, in which: y

Figure '1 is a greatly enlarged sectional view through my improved surface coating.

Figure 2 corresponds to Fig. l but the several coatings are not illustrated in proportionate thicknesses and the several indices of refraction and partial reflection are indicated; and

Figure 3 is a vector diagram of the amplitudes of the light or other radiant energy reflected at the boundaries of the various media constituting the coating of Figures 1 and 2, and including the glass or other transparent support for the coat- (ci. ss-i) ing on the one hand and air or other medium to which the iight passes on the other hand.

Fig. l illustrates what is at present the preferred form of the invention although many modifications thereof are quite practical.

In the form of the invention shown in Fig. l, the glass It is first coated with a very thin evaporated lm l i of what I believe is thorium oxiuoride. This material is prepared by heating l thorium uoride ThFrHzO in a platinum crucibie in a vacuum or in an inert gas. I prefer to use the term inert atmosphere to include a vacuum or an inert gas. When the thorium fluoride is prepared from thorium nitrate and hydrouoric acid, it takes the tetrohydrated form given above.

When this thorium uoride is heated to 200i C. it gives 01T 3 molecules of water forming the monohydrate. If this monohydrate is heated in air to a red heat, it forms thorium oxide and hydrogen iluoride. If, however, the thorium iiuoride monohydrate is heated to substantially red heat in a vacuum, the following reaction takes place 'I'hF4H2Ol-heat=fI'hOFz+2I-EF. While there is reason to believe that the Y above compound ThOFa is formed, it is possible that the water of crystallization is merely driven 0E, leaving ThF, and I do not wish to be restricted to thorium oxi-fluoride when the resulting compound may 'be thorium uoride. i

The compound comprising thorium and uorine prepared in thismanner is placed in a platinum boat and heated to about 1000 degrees C. in a vacuum (preferably lower than 1 micron). At this temperatureit melts and evaporates and it may be evaporated onto the optical surface to 'an appropriate thickness. The evaporated iilm of this material after it has been baked for several hours at a temperature of degrees C. or

over the other types of lms in that it repels wav ter, it is extremely hard, it has a very low surface friction, it is not soluble in pure water, salt water, or in alkaline solutions, it forms a very good bond with the glass and it is clear and transparent without appreciable absorption in the visible spectrum. Certain types of glass are hydroscoplc and this coating material seals the glass against moisture. Although as indicated above, it is not absolutely certain that the material thus produced is thorium oxi-iluoride, for the purposes of the present specification, I will refer to it as sucln Before baking. the thoriumoxi-uoride has an index of refraction of approximately 1.45 which is toc high in itself to be satisfactorily used as a somewhat higher, has a number of advantages reflection reducing coating. This coating is placed on the glass first because it has a very good adhesion to both glass and the subsequent coatings to be placed thereon. n the surface of this first layer Il there is placed a second layer I2 of Zinc v sulphide which is likewise very thin. In referring reflection would not be sufficient for most purposes, while by adding the high index coating the reflection may be reduced to values much lower than is obtained from a 1/4k of magnesium, fluoride. The effect of the high index layer in reducing reflection is best shown by means of the vector diagram Fig. 3.

Although I prefer zinc sulphide as the high index layer, thereare other materials which may be used, such for example as lead fluoride, copper iodide, bismuth fluoride or bismuth oxiuoride.

All of these materials are insoluble .in water. 1 When using these high index materials, the first thorium oxi-iluoride layer is not as essential since many of these materials adhere quite well directly to the glass. Bismuth uoride is particularly good in this respect and a satisfactory lm may be produced in only three layers if it is `so desired. Lead fluoride may also be used although it is poisonous to work with. In. this casewe would have bismuth fluoride contiguous to the glass, magnesium fluoride over this and nally a covering layer of thorium oxi-uoride.

Over the film of zinc sulphide I2 there 'is evaporated a second film of thorium exi-fluoride I3 which is likewiservery thin. The purpose of this film is to improve the adhesion of the next layer itl-the fourth, which is magnesium uoride and which does not adhere too well tothe zinc sulphide. The thorium exi-fluoride adheres very well to both the zinc sulphide and the magnesium fluoride. fluoride, isl a little less than All thick for -the light for which the minimum reduction in reection is desired. The thickness should be enough l ss than AA to allow for the application of the s bsequent very thin layer producing a total effctive Yoptical thickness of substantially Mix. rli'l'iis magnesium fluoride is evaporated onto the siu'face in the same manner as the previous coatings. Although I have specifically referred to magnesiumfluoride as my preferred material 'which is particularlydesira'ble because it is chemifzally durable and convenient tov use, there are 'other substances which can be used. Some are more desirable in some respects and others in otherrespects, but have disadvantages'. For example, calcium fluoride may be used as described in Cartwright Reissue Patent No. 22,076, or calcium aluminum hydroxide fluoride may be used as described and claimed in my application, Ser. No. 408,807,A filed August 29, 1941, orA calcium fluoride or the evaporation product of a mixture of calcium uori'de and aluminum oxide may be used as described and claimed in my application, Ser. No. 248,815 filed July 31. 1940 (now Patent No. 2,338,233). The fifth layer I5 is again thorium exi-fluoride and is evaporated to such a thickness as to result in the minimum overall reection for the color desired. This outer coating ox thorium oxi-fluoride very greatly increases the mechanical and chemical durability of the low reflecting coating. It is so durable that tests have shown it to stand a saturated solution of boiling salt Water or boiling 10 percent sodium hydroxidesolutlon without damage tothe urf'ace. The magnesium fluoride coating alone is ncapable of withstanding either of these solutions and the glass surface is incapable of withstanding the sodium hydroxide solution. The thorium oxi-fiuoride coating is also very hard and has such low surface friction that it is not scratched by any of the normal cleaning processes.

The multi-layer film is normally baked for an Y hour or more at a temperature o f vabout 'Z0 delgrees C. in order to secure maximum adhesion and hardness.

In Fig. 2 the relative thicknesses of the layers are not drawn to scale and this gure shows the indices of refraction of various layers, the relative indices of refraction of the various layers and the calculated values of the reflected amplitudes at the six. boundaries. The amplitudes are calculated from the equation a N1/No'1 N1/N+i A complete vector'diagram of the ilve layer film is shown in Fig. 3. 'I'his method of analysis is described in my technical paper entitled A new dichroic reflector and its application to photocell monitoring systems published in'the Journal of the Society of Motion Picture Engineers for January 1942. The amplitudes shown in Fig.v

and tz-. a); etc.

l where A is the wave llength of the light `and N is The low index layer Il, of magnesium the index of refraction of the layer. R1, Rz, Ra,

and R4 (Fig. 3) are vthe resultants obtained. by vectorial addition (by the parallelogram method) of the separate amplitudes as shown Re is the final resultant audits length represents' the final reected amplitude. of the reflected intensity. l

The zinc sulphide, the magnesium fluoride and -the thorium oxi-uoride were all evaporated from platinum containers. ,'I have found platiy num to be the best material for this purpose be-v cause it ischemically inert, it has a very high melting temperature, it has a very low vapor pressure. The magnesium fluoride and the thooxl-uoride Wet. the platinum which helps to transfer the heat'to the materials. 'I'he zinc sulphide sublimates at about 1100 degrees centi'- grade and goes directly from la solid to a vapor.

I claim as my invention: Y 1. In combination with a radiant energy transparent support medium, a transparent coating disposed on a surface of said support medium for reducing reflection of radiant energy from said surfaceinto an adjacent medium, said coating comprising a plurality of continuous layers, one of said layers being of material of higher -index of refraction than the support medium, another of said layers being of material of lower index of refraction than the support medium, and a third layer being of a compound including thorium and fluorine, said low index layer'having an effective optical thickness less than one-quarter of a particular wave length of radiant energy, said Rs2 is the na-l value 2. The invention as set forth in claim 1 wherevin said high index material is a fluoride of bis: muth. f

3. In combination with a radiant energy transparent support medium, a transparent coating layers being of a diii'erent material characterized by relatively low index of refraction, said layers being of such thicknesses that the sum of the disposedgon the surface of said support medium for reducing reflection of radiant energy from said surface into an adjacent radiant energy transparent medium, said coating comprising a layer of an alkali earth fluoride having a thickness less than 1A of a desired wave length of radiant energy and a superposed layer of a compound of thorium and uorine of Isuch thickness that the sum of the amplitude vectors representing the reflection from'the boundaries., between said layers-and between said coating and said mediums, respectively, is a minimum for said desired wave length.

4. In combination with a radiant energy transparent support medium,4 a transparent coating disposed on the surface of said support medium for reducing reflection of radiant energy fro-m said surface into an adjacent` radiant energy transparent medium, said coating comprising a layer of magnesium fluoride having a thickness less than A of a desired wave length of radiant energy and a layer of a compound of thorium and iiuorlne of such thickness that the sum of the amplitude vectors representing the reflection from the boundaries between said layers and between said coating and said mediums, respective- 1y, is a minimum for said desired wave length.

5. The invention as set forth in claim l wherein a layer of a compound of thorium and uorine is contiguous to one of said mediums.

6. The invention as setforth in claim 1 wherein a layer of a compound of thorium and fluorine is contiguous to one of said mediums and another oi such layers is'disposed between said high and low index layers.

7. In combination with a light energy transparent support medium, a transparent coating disposed on the surface of said support medium for reducing reiiection of light energy from said surface into an adjacent light energy transmitting medium, said coating comprising a plurality of superposed layers, said layers comprising, respectively, a compound of thorium and fiuorine, zinc sulphide, a compound of thorium and uorine, magnesium uoride, and a compound of thorium and fluorine, in the order listed from said support medium to said adjacent medium, said layers being'of such thicknesses that, the sum of the amplitude vectors representing the reflection from the boundaries between said layers and between said coating and said mediums are substantially a minimum for a desired wavelength.

8. In combination with a radiant energy transparentsupport medium, a transparent 'coating t disposed on the surface of said support medium for reducing ref action of radiant energy from said surface into an adjacent radiant energy transparent medium, said coating comprising a plurality of superposed adherent thin layers, a first of said layers comprising a compound of bismuth and fiuorine and having a relatively high index of refraction, a second of said in said rst layer is next adjacent said support' medium and contiguous to said second layer, and a third of said layers characterized by. great chemical and physical resistance is contiguous to said second layer and to saidadjacent medium.

11. The invention as set forth in claim 8 wherein a third of said layers is a compound of thorium and fluorine. Y

l2. The invention as set forth in claim 8 whereg in a third of said layers is a compound of thorium and fiuorine contiguous to said adjacent medium.

13. The invention as set forth in claim 8 wherein said second of said layers is analkaline earth compound and a third of said layers is a compound of thorium and luorine contiguous thereto and to said adjacent medium.

14. The invention as set forth in claim 8 wherein said second layer is an alkaline earth compound having a thickness less than S of a desired wave length of radiant energy and wherein a third of said layers is a compound of thorium and fiuorine contiguous thereto and to one of said mediums.

15. In combination with a radiant energy rtransparent support medium, a transparent coating disposed on the surface of said support medium for reducing reection of radiant energy from said surface into an adjacent radiant energy transparent medium, said coating comprising a plurality of superposed adherent thin layers, a first of said layers comprising bismuth uoride, a second of said layers comprising magnesium iiuoride, and a third of said layers comprising a compound of fluorine and thorium, said layers being contiguous to each other and to said mediums in the order listed from said support medium to said adjacent medium, said layers being of such thicknesses that the sum of the amplitude vectors representing the reflection fromthe boundaries between said layers and between said coating and said mediums are substantially a minimum for a desired wavelength.

16. In combination, a transparent support, a layer of an alkali earth fluoride having a thickness than than A wave length of light on the surface thereof and a layer of thorium oxi-uoride 'on the surface of said layer of alkali earth uoride of such thickness that with the layer of alkali earth fluoride it has a total eiective thickness approximating y; of a Wave length of light.

17. In combination, a. transparent support, a layer of magnesium fluoride having a thickness less than $41 wave length of light on the surface thereof and a layerof thorium oxifiuoride on the surface of said layer of magnesium fluoride of such thickness that with the layer of magnesium iiuoride it has a total effective thickness of approximating t4; of a wave length of light.

18. In combination, a transparent support, a very thin layer of thorium oXi-iiuoride on the surface thereof, a layer of an alkali earth uoride having a thickness less than A wave length of light on the surface of the thorium om'iiuoride and a layer of thorium oid-fluoride on the surface of said layer of alkali earth fluoride of such thickness that with the layer of alkali earth uoride it has a total eective thickness approximating V4 of a wave length of light.

19. In combination, a transparent support, a very thin layer of thorium exi-fluoride on the surface thereof, a layer of magnesium iluoride having a thickness less than 1A wavelength of light on the surface of the thorium oxi-iluoride and a layer of thorium oxi-fluoride on the surface of said layer' of magnesium iiuoride of such thickness that with the layer of magnesium iluoride it has a total eiective thickness approximating V1,

of a wavelength of light,

20. In combination a. transparent support, a

very thin and closely adherent layer of thoriumY oxi-iuoride on the surface thereof, a layer of material having a'high index of refraction on the surface of said layer of thorium oxi-uoride, a second layer of thorium oxi-iuoride onthe surface of said layer of material of high index of rethat with thelayer of alkali earth fluoride it has a total eifective thickness-approximating V4 oi a wave length of light. v

22. In combination with a light energy transparent support medium, a transparent coating disposed on the/surface of said support medium rium and iluorine, said layers being of such thicknesses that the sum of the amplitude vectors representing the reection from the boundaries bee tween said layers and between said coating and fraction, a layer of an alkali earth iluoride having very `thin and closely adherent layer of thoriumv oxi-uoride on the surfacethereof, a very thin layer of material having a high index of refraction on the surface of said layer of thorium oxiuoride,`a second layer of thorium oxi-uoride on the surface of said layer of material of high in- Y dex of refraction, a layer of an alkali earth iluoride having a' thickness less than l wave length of light on the surface of the thorium exi-fluoride and a layer of thorium oxifuoride on the said layer of alkali earth fluoride oi' such thickness `said mediums, respectively, is a minimum for a desired wavelength. y

23. The invention as Set forth in claim 22 whereina layer of a compound of thorium and iiuorine is contiguous to one of said mediums.

24. The invention as set forth in claim 22 wherein a layer of alcompound of thorium and nucrine is centiuous tc each ci said 25. The invention as set forth in claim 22 wherein a layer of a' compound of thorium and iiuorine is contiguousto one of said mediums and 'another such layer is disposed between said pair of layers.

26. The vinvention as setforth in claim 22 l whereina layer of a compound of thorium and pair of layers.

' .f GLENN L. DIMMICK. 

